There has been proposed a technology of forming a plurality of density patches having different gradations on a photosensitive drum and correcting each gradation characteristic of each density patch on the basis of a detection result of each density patch on the photosensitive drum by an optical sensor. According to the related-art technology, a gradation characteristic of image data, which is a printing target, is corrected on the basis of a detection result of a pattern image for detection, which is to be detected by the optical sensor, by the optical sensor. Also, according to the related-art technology, the pattern image for detection in a low gradation area is configured by a plurality of isolated unit images, and a size of the unit image becomes larger as the density increases.
However, according to the related-art technology, when forming the pattern image for detection in the low gradation area on the photosensitive member, the unit image becomes small with respect to a base of the photosensitive member, so that an S/N ratio of a detection signal decreases upon the detection by the optical sensor.